1. Field of the Invention
The present invention relates to an optical phase locked loop circuit for synchronizing the phase of a light source in optical communications or optical measuring instruments.
2. Background Art
FIG. 2 shows an optical phase locked loop circuit in the prior art. In this figure, the optical circuit comprises a stability controller 10, a master laser 11, an optical coupling device 12, a photodetector 13, a high-pass filter(HPF) 14, a phase comparator 15, a signal generator 16, a low-pass filter (LPF) 17, an optical frequency controller 18, a slave laser 19, and an optical branching device 20.
The stability controller 10 controls the master laser 11 in order to stabilize an optical power P.sub.M and an optical frequency f.sub.M of a coherent light 11A generated by the master laser 11. The coherent light 11A is supplied to the optical coupling device 12. This device 12 couples the coherent light 11A with an output control light 20B. The light 20B is separated from an output light 19A of the slave laser 19 by the optical branching device 20 and has an optical power P.sub.S and an optical frequency f.sub.S.
The photodetector 13 heterodynes the coupled light 12A received from the coupling device 12, and outputs a frequency difference signal 13A, a proportional signal 13B and a proportional signal 13C. The amplitude of the difference signal 13A is proportional to the product between the square root of the optical power P.sub.M of the coherent light 11A and the square root of the optical power P.sub.S of the output control light 20B. The frequency of the signal 13A is the difference frequency .DELTA.f=f.sub.S -f.sub.M between the frequency f.sub.S of the output control light 20B and the frequency f.sub.M of the coherent light 11A. The amplitude of the signal 13B is proportional to the optical power P.sub.S of the light 20B. The amplitude of the signal 13C is proportional to the optical power P.sub.M of the coherent light 11A.
The optical power P.sub.S of the output control light 20B varies in correspondence to the output frequencies of the slave laser 19 controlled by the optical frequency controller 18, therefore the signal 13B contains a low frequency component corresponding to the time-dependent variation of the optical power P.sub.S. The optical power P.sub.M of the coherent light 11A is stabilized by the stability controller 10, therefore the signal 13C constitutes a direct current component which does not vary with time.
The HPF 14 receives the signals 13A,13B,13C generated by the photodetector 13, and transmits the difference component .DELTA.f of the difference frequency signal 13A to the phase comparator 15 by filtering the low frequency component and the direct current component included in the signals 13B,13C.
The phase comparator 15 compares the phase of the difference frequency signal 13A and the phase of a signal 16A having an amplitude E.sub.H and a frequency f.sub.H. If there is a phase difference caused by a difference in the frequencies between the signal 13A and the signal 16A(i.e. f.sub.H .noteq..DELTA.f), the phase comparator 15 outputs a signal 15A having an amplitude which is proportional to the product between the square root of the amplitude (P.sub.M.P.sub.S) of the difference frequency signal 13A and the amplitude E.sub.H of the signal 16A, and having a frequency which constitutes a difference frequency component .DELTA.F=f.sub.H -.DELTA.f between the signal 13A and the signal 16A. If there is a phase difference, even if the frequency of the difference frequency signal 13A is the same as that of the signal 16A(i.e. f.sub.H =.DELTA.f), the phase comparator 15 outputs a signal 15A having an amplitude which is proportional to the product between the square root of the amplitude (P.sub.M.P.sub.S) of the difference frequency signal 13A and the amplitude E.sub.H of signal 16A, and being in correspondence to the phase difference between the difference frequency signal 13A and the signal 16A.
The output signal 15A of the phase comparator 15 is supplied to the optical frequency controller 18 after filtering noise components by the LPF 17. The optical frequency controller 18 generates a signal 18A, which is supplied to the slave laser 19 in order to vary its optical frequency, so as to make f.sub.H equal to .DELTA.f(i.e. f.sub.H =.DELTA.f), and the phase difference between the difference frequency signal 13A and the signal 16A to become zero.
The slave laser 19 outputs the light 19A of a controlled optical frequency f.sub.S in accordance with the signal 18A supplied from the optical frequency controller 18. The light 19A is guided to the optical branching device 20. In this device 20, the light 19A is separated into an external output light 20A and the output control light 20B. The output control light 20B is guided to the optical coupling device 12.
In the configuration shown in FIG. 2, The amplitude of the difference frequency signal 13A varies when the optical power P.sub.S of the output control light 20B varies with time, because the amplitude of the frequency signal 13A supplied to the phase comparator 15 via the HPF 14 is proportional to the square root of the optical power P.sub.S of the output control light 20B.
Therefore, the amplitude of the output signal 15A from the phase comparator 15 varies even if f.sub.H is made equal to .DELTA.f (i.e. f.sub.H =.DELTA.f) and the phase difference between the difference frequency signal 13A and the signal 16A is eliminated. This amplitude variation of the output signal 15A from the phase comparator 15 results in the variations in the optical frequency of the output light 19A from the slave laser 19.